Heretofore, among various thermal transfer recording methods, a heat-sensitive sublimation-type transfer printing method, and a heat-sensitive melt transfer printing method have been widely employed. The heat-sensitive sublimation-type transfer printing method is such that an image is produced, by the use of a heating device such as a thermal head whose heat generation is controlled according to image information, on a thermal transfer image-receiving sheet by transferring a sublimable dye, coloring material, contained in a sublimable dye layer provided on a thermal transfer sheet (dye donor sheet).
In this heat-sensitive sublimation-type transfer printing method, the amount of a dye to be transferred to form each dot can be controlled by the application of heat in an extremely short time. Further, a dye, which is excellent in transparency, is used as a coloring material, so that the image formed is very clear, and, at the same time, excellent in the reproducibility and gradation of half-tone. For this reason, there can be obtained, by this printing method, extremely precise images with high quality, comparable to full-colored images obtainable by photography using silver salts.
As a thermal transfer image-receiving sheet (hereinafter sometimes referred to as "image-receiving sheet") for use in such a heat-sensitive sublimation-type transfer printing method, a sheet prepared by forming a dye-receptive layer on a substrate sheet is generally used.
As the properties required for this image-receiving sheet, not only high printing sensitivity but also resistance to the curling of the image-receiving sheet itself (curling resistance) is important. For this reason, when the image-receiving sheet is prepared in sheet form, there has been adopted a method in which the curling of the image-receiving sheet is prevented by symmetrically providing a plastic sheet or the like on both surfaces of the substrate sheet such as a plastic sheet or paper, or a method in which curling resistance is imparted to the image-receiving sheet by providing a curling-preventing layer made from a resin or the like on the back surface of the substrate sheet.
In order to prevent curling, there has also been employed, for example, a method in which a polyethylene or polypropylene layer is provided on one surface (back surface) of a substrate, opposite to the surface on which a coloring-material-receptive layer is provided, or on both surfaces of a substrate. However, when the image-receiving sheets obtained by these methods are stored in a high-temperature or high-humidity environment, they can not always show satisfactory curling resistance. Specifically, in the case of an image-receiving sheet prepared by providing a polyethylene layer as the curling-preventing layer, although it shows excellent curling resistance when stored at high humidities, the degree of curl tends to be increased when it is stored at high temperatures. On the contrary, in the case of an image-receiving sheet prepared by providing a polypropylene layer as the curling-preventing layer, although it shows excellent curling resistance when stored at high temperatures, the degree of curl tends to be increased when it is stored at high humidities.
Further, in the case where a resin layer having minute voids is provided between a substrate and a coloring-material-receptive layer so as to obtain improved printing sensitivity, a method in which a resin layer having minute voids is provided also on the back surface of the substrate may be employed in order to prevent the curling of the image-receiving sheet. However, although the effect of preventing curling can be obtained by this method, the production cost is remarkably increased. For this reason, this method has been unsatisfactory from the economical point of view.
The present invention has been accomplished in view of the aforementioned background. An object of the present invention is therefore to provide an image-receiving sheet capable of showing excellent curling resistance even in a high-temperature or high-humidity environment although the back and face of the image-receiving sheet are not symmetrical in structure, by forming a layer useful for preventing curling, using a relatively inexpensive general-purpose resin on the back surface or both surfaces of a substrate which is the core of the image-receiving sheet.